1. Field of the Invention
This invention relates to a coating composition based on waxes or wax-like compounds or air-drying oils or alkyd resins.
2. Discussion of Related Art
Metallic substrates are provided with film-forming coatings to protect them against corrosion. Several different types of coating are used for this purpose, depending on requirements. This applies in particular to steel substrates, as encountered for example in the automobile industry. Because of their design, automobile bodies, for example, have a number of awkwardly situated cavity voids and seams which are inadequately coated during normal lacquering processes and which are normally coated with waxes or wax-like compositions. These compositions are required to show very good wetting behavior and adhesion behavior with respect to the substrate to be coated, a good water (vapor) barrier effect and, during application, good creep behavior so that they are able under the effect of capillary forces to penetrate into the fine capillary cavities or seams and to displace any layers of water present. As mentioned above, these coatings are used in awkwardly situated places so that their mechanical strength, hardness and abrasion resistance generally have to meet only minimal requirements. These compositions are also commonly referred to as "cavity sealing compounds".
Waxes and/or fats or oils as binders have long been successfully used for these anticorrosive coatings. The waxes used may be vegetable waxes (for example carnauba wax, montan wax), animal waxes, mineral waxes, more particularly petrochemical waxes (petrolatum, oxidized petrolatum, paraffin waxes or microwaxes), chemically modified waxes or even synthetic waxes. In addition, drying oils or chemical modifications thereof, such as long-oil alkyd resins, for example, either on their own or in combination with the above-mentioned waxes, and also synthetic hydrocarbon resins may be used.
The anticorrosive coatings contain corrosion inhibitors as a further constituent. The corrosion inhibitors may be, for example, inorganic pigments, organic inhibitors such as, for example, amines or salts thereof or metal salt dispersions. One representative of metal salt dispersions, the so-called superbasic calcium salts of organic sulfonic acids, are used particularly frequently for anticorrosive coatings because they apparently form films highly impermeable to the diffusion of water vapor and, by virtue of their basic nature, are capable of neutralizing corrosion-promoting acid traces from the environment. A review of such metal salt dispersiens can be found, for example, in R. M. Morawek, Modern Paint Coatings, 69 (1979), 49-51. These metal salt dispersions are also commercially known by the name of "SACI" (severe atmospheric corrosion inhibitors).
The superbasic calcium salts of organic sulfonic acids are prepared, for example, by stirring calcium oxide and/or hydroxide into alcoholic or aqueous/alcohol solutions of sulfonic acids of paraffin hydrocarbons and subsequently introducing carbon dioxide, calcium oxide or hydroxide being used in such a quantity that a considerable excess of calcium oxide or hydroxide is left in the dispersion. Superbasic calcium sulfonates in the context of the present invention and their production and use as lubricants or rustproofing agents are described, for example, in DE 19 19 317 or in EP 405 879.
Other typical constituents of the formulations are pigments, including for example so-called anticorrosive pigments, and/or fillers, such as chalks (both natural ground and also finer precipitated chalks), grinding or dispersing aids in the form of fatty acid salts of organic amines and, where drying oils and/or alkyd resins are present, antiskinning agents and so-called dryers or siccatives which catalyze the oxidative crosslinking of the drying oils or alkyd resins.
The anticorrosive coating compositions may be sprayed on through special nozzles in the form of solutions or dispersions in organic solvents, such as hydrocarbons (gasolines) or chlorinated hydrocarbons, as described for example in DE-C-27 11 596. To avoid volatile organic constituents, a so-called "flood wax process" is proposed in DE-A 27 55 947. In this process, the cavities are "flooded" in a first step with molten wax-like corrosion-preventing materials. In a second step, the excess, still liquid material is allowed to run off from the cavities.
The anticorrosive coating compositions should preferably have a low viscosity to ensure that the metal surface, including the capillary cavities, are completely covered, even when the coating is applied in a thin layer. Accordingly, almost all compositions for these anticorrosive coatings, except the compositions for the flood wax process, are dilute solutions in readily volatile organic solvents or dispersions in organic solvents or water.
The solventless systems are, for example, anticorrosive oils similar to the compositions described in DE 19 19 317 consisting of a superbasic calcium sulfonate, mineral oil and a gasoline fraction ("Stoddard" solvent). These oils are generally sprayed at room temperature and form an oily, generally tacky film. After cooling, the flood waxes form a more or less solid film. In the case of the solventless products based on alkyd resins or drying oils, such as linseed oil varnish for example, gradual oxidative crosslinking of the film begins after application. In the case of the solvent-containing dispersions or solutions, film formation generally takes place purely physically after evaporation of the solvent.
The disadvantage of all the compositions for anticorrosive coatings described in the foregoing lies in their temperature sensitivity and in their very low mechanical stability immediately after application of the coating. In automobile construction, the coatings in question are applied at an early stage in the assembly process. In many cases, the coatings described above tend to "run" after drying, i.e. above all on further exposure to heat in the assembly process, the anticorrosive coatings already applied become so thinly liquid that the material runs or drips off the body, resulting in leaking and soiling of the bodies and of conveyor belts and assembly belts and work stations in the assembly shops. In addition, so-called "sump" formation can occur, i.e. the material which has run off collects in low-lying seams and cavities of the bodies. This interferes with subsequent assembly work.
There has been no shortage of attempts to overcome this deficiency. Thus, EP 259 271 describes the addition of vinyl polymers. These polymers are produced in situ in the anticorrosive composition by radical polymerization of vinyl monomers, such as methacrylic acid for example. Although this process reduces dripping after application of the composition to the body, the viscosity of the compositions is so high that only very dilute solutions or dispersions in organic solvents can be used. However, this is undesirable on ecological grounds.
DE-A-28 25 739 proposes a solventless anticorrosive composition. This composition consists essentially of a petroleum sulfonate complex of calcium and/or magnesium in a non-volatile diluting oil, oxidized petrolatum, microcrystalline wax, anticorrosive pigment and a thermoplastic hydrocarbon resin. Although this composition is free from volatile solvents, the material has to be heated to temperatures of 135.degree. C. to 150.degree. C. to be applied by spraying. Apart from the considerable effort involved in the production and application of the material, difficulties also occur in the uniform wetting of the cold substrates, such as automobile bodies for example. This can only be avoided by expensive heating of the bodies.
U.S. Pat. No. 4,386,173 similarly proposes anticorrosive compositions consisting of petroleum sulfonate complexes, waxes, pigments, fillers, non-volatile oils and an epoxy resin elastomer. These materials also have to be sprayed at temperatures of at least 135.degree. C.
Accordingly, the problem addressed by the present invention was to provide a solventless or low-solvent anticorrosive coating composition which would show sufficiently low viscosity to be able to be sprayed at room temperature in conventional plants. In addition, the coating would be able to be converted by simple means into a form which would reliably prevent running and soiling of body parts and plant components.